In the U.S. Pat. No. 4,638,754, to Tornay, issued Jan. 27, 1987, there is disclosed a vessel hull and construction method, which is said to be particularly suitable for vessels of double hull construction, such as gas tankers with almost complete double hulls, and oil and product tankers with double bottoms. The disclosed system is said to be suitable for construction of commercial and naval ships and barges, and to be adaptable to provide any arrangement of decks and bulkheads within the hull.
In the construction system disclosed in the Tornay patent, the plating of the outer hull and bulkheads is constructed of standard-sized steel plates, usually about 8 feet by 40 feet, which are rolled to a slight cylindrical curvature, the axis of which is parallel to the longer edge. On the shell of the vessel, the plates are arranged with their long edges longitudinal. The plates are arranged with the curvature inwards or outwards, depending on the direction of the highest local load. The curved plates are then provided with a reverse bend or recurve along the midline between the long longitudinal edges. The short transverse edges and the long longitudinal edges are butt-welded together. To the midline of the plates at the recurve are welded longitudinal steel girders such as L or T-beams which match the recurved portion of the curved plates and are of sufficient depth and section modulus to span the distance between transverse bulkheads or deep webs. The local pressure loading on the plate causes a tensile or compressive stress, depending on the direction of the load, uniformly across the thickness of the curved plate in the same way that a cylindrical pressure vessel is stressed by an internal pressure. Because the plating is not subjected to local panel bending, closely spaced primary stiffeners are said to be not required. In addition to efficiently absorbing local pressure forces such as transverse membrane stresses, the curve and reverse curve also are said to provide stiffness in the longitudinal direction needed to resist compressive buckling caused by longitudinal hull bending stresses. An additional efficiency is said to be obtained from the girders along the midline of the curved plates, and thus contribute to the principal section modulus of the hull needed to resist longitudinal bending.
Such details of the construction as orientation of each module while being constructed, the method and means for performing the welding of module elements to one another, and of modules to one another, jigs or fixtures useful during module fabrication or joining, and module handling and manipulation apparatus and techniques do not appear to be presented in the Tornay patent, leading one to conclude that no non-conventional methods or means for construction were contemplated.
In a vessel hull, the ability of an unreinforced curved steel to resist pressure forces, bending, and buckling is determined by well known and classic criteria and is a direct function of plate width, length and thickness and an indirect function of radius of curvature.
Some flat plate tanker designs exist in which there is no transverse structure reinforcing the inner and outer hulls except for transverse bulkheads. The elimination of transverse structure in such designs is not therefore a new idea. However, the use of curved plate results in a buildup of excessive transverse forces at the lower corners (bilges) of the vessel which would normally require the installation of transverse deep webs, as contemplated in the Tornay patent.
Details of the design of a curved plate vessel which would eliminate the need for deep webs are not disclosed in the Tornay patent, nor are details which would eliminate the need to recurve the curved plate along its midline.
The use of a flat plate duct keel and flat plate upper corner box girders which permit the use of standardized curved plate modules in a wide range of double hull tanker sizes is not disclosed in the Tornay patent.
The use of a curved plate midbody in combination with flat plate bow and stern ends of a double hull tanker is not disclosed in the Tornay patent.
Shipfitting is the most technically demanding and time-consuming element of subassembling vessel hull sections. Shifting, holding, bending and trimming of plates and other elements is necessary so that welding can be satisfactorily accomplished and so that subassemblies can be assembled to one another to provide a hull that meets specifications.
The techniques of electroslag/electrogas welding generally require the use of a continuous cooling shoe on the back-sides of T-joints while welding is being accomplished. In prior art construction techniques using electroslag/ electrogas welding, efficiency has been penalized due to the time needed to set-up and provide cooling water plumbing to the cooling shoe.
Shipfitting and welding of vessel hulls as heretofore practiced in even the most highly rationalized construction facilities has necessitated that the construction workers have a lot of training and possess a wide range of skills. Increasing scarcity of trained workers possessing such a broad range of skills has driven-up labor costs to the point where vessel hulls cannot be profitably fabricated in many areas.
Conventional fabrication techniques also have made extensive use of temporary shipfitting devices, many of which are welded-on in normal practice, necessitating a considerable amount of surface preparation and painting after the devices are disconnected from the work.
Also in the prior art are assembly lines for flat panels. A typical panel assembly line comprises a long conveyor along which are mounted devices for assembling and welding steel sheets into panels each weighing, for instance, from 10 to 50 tons. Flat panels, when assembled to provide hulls, require the use of transverse framing crossing longitudinal framing.
If vessel hulls are constructed in modules that are sequentially connected to form the hull, and an attempt is made to do so on land, the land crane that would need to be used is expensive, and, should it become no longer needed due to eventual curtailing of the project, can be expected to have lost considerable value.
Land-based assembly might use a plurality (e.g., from six to ten) mobile cranes on pile-supported tracks. These are expensive to buy, install and maintain.
Two shipyards (neither of which is still operating), known to the present inventors, formerly used modular assembly and erection procedures for constructing vessel hulls.
One of these, the Erie yard of Litton Industries, built 1000-foot long ore carrier midbodies. (A tanker/carrier "midbody" is the main portion of the length of such a vessel, but for the bow and stern sections, that is of substantially constant transverse cross-sectional size and shape.)
At the Erie yard, midbody modules were each built in an upended spatial disposition (one which will be referred to herein as being "vertical"). These modules were built in the extreme end of the same graving dock that was used for joining the modules to one another and for joining purchased bow and stern units to the midbody. The modules weighed 600 tons each, and an electro-hydraulic jacking device was used for forming the individual upended modules over into a horizontal orientation for serial joining to form the midbody. The hydraulic jacking device, one of the largest ever made, was expensive to purchase, install and maintain. Failure of the device would have posed a serious threat to human life, and to the graving dock, which was the most expensive and indispensable facility at the Erie yard.
(Graving docks, such as the one at Erie, use stationary civil construction including pilings, concrete, fill, and piping embedded in the concrete of the dock floor. Pumps are continuously operated for relieving the dock, through the pipes, of water seepage through the dock floor. A graving dock functions much as if it were a bathtub with a gate at one end to let a vessel in and out. The level of the floor is sufficiently below the level of the adjoining body of water to permit the vessel being built or dry-docked to float clear of the blocking on the floor of the dock when the graving dock is filled with water by opening valves connected to that adjoining body of water. The concrete floor of a graving dock typically is supported by piling that is adequate to support the full weight of the vessel when the graving dock is empty of water. The perimeter wall of a graving dock, is constructed high enough to keep water out (except for seepage), at the highest tide for which the facility is designed to encounter.)
The module assembly area at the Erie yard was not accessible to heavy-lift floating derricks. Therefore, each subassembly had to be lifted using a land crane, often reaching to or beyond the maximum for which it was designed. The crane lifting and reaching capabilities acted as limits on the size and weight of individual midbody modules which could be fabricated at the Erie yard.
The fact that the graving dock at the Erie yard was constructed in place and fixed in its location meant that it had limited residual value, it could not be floated away and used elsewhere once there was no longer need for it at Erie.
One other shipyard at which modular assembly and erection procedures were used for constructing carrier/tanker vessel hulls, was the Arandal yard in Gotverken, Sweden. At the Gotverken yard, oil tankers were built in a graving dock. One end of the graving dock was located inside an enclosed building. Modules were assembled horizontally, one after another, at a single station located in the building, and then moved out into position in the open end of the graving dock. Probably the movement was accomplished on rollers. Because the modules were constructed horizontally, there was reduced accessibility for steel assembly and piping installation, compared with vertical construction, where a module being constructed is open at the top rather than at the side.
Caissons are commonly used for underwater work, e.g., for the construction of bridge piers. It is believed by the present inventors that caissons may have been used in the past for joining vessel hull sections.
Large underwater grids have been used under floating dry docks to stabilize them as ships were transferred off a floating dry dock onto land, or from land onto a floating dry dock. A ship construction facility known as the Ingalls launching pontoon uses this technique.
The task of inspecting and maintaining conventional double-bottomed vessel hulls incorporated in carriers for bulk fluids and/or particulate material such as oil, liquified gases, chemicals, ore and grain is difficult. With traditional egg crate-like stiffening structures provided longitudinally and transversely between the bottoms, the process is one of climbing-over, ducking-under and squeezing through girders and plates in a constantly couched position, with poor ventilation (unless "moon suits" are worn), poor footing, little to hold onto for support, while needing to carry illumination means, and trying one's best to locate anomalies in surfaces the expected color of which is itself dirt colored or rust colored.